Breast cancer is the second most frequently occurring malignancy in U.S. women, and a more thorough understanding of its pathogenesis should enable the development of improved tools for detection, prognosis determination, and treatment. Overexpression of the cell cycle regulatory protein, cyclin E, is frequently observed in human breast tumors, and high cyclin E is proposed to be a marker of poor prognosis cancers. Whether deregulation of cyclin E contributes directly to breast tumorigenesis is unknown. The overall goals of the proposed experiments are to understand how deregulated cyclin E activity functions during mammary tumorigenesis and to identify the key cellular barriers against cyclin E-associated breast cancer. In addition to high cyclin E expression, reduced expression of the cyclin-dependent kinase inhibitor p27Kip1 has also been found to correlate with poor prognosis in breast cancers. Using three-dimensional mammary epithelial cultures, I found that cyclin E overexpression cooperates with decreased p27 expression to produce hyper-proliferative mammary epithelial structures in vitro. In Specific Aim 1, I will elucidate the basis for this cooperation. I recently developed a knockin mouse containing mutations at two major cyclin E phosphorylation sites regulating its binding to Fbw7/SCF ubiquitin ligase complex. Tissues from these animals contain markedly elevated cyclin E protein levels and kinase activity and are hyper-proliferative, including the mammary epithelia. Based on results of my preliminary studies, I hypothesize that the p27 and p53 tumor suppressors inhibit the oncogenicity of deregulated cyclin E. In experiments proposed in Aims 2 and 3, I will develop new animal models to understand how loss of normal p27 and p53 function, respectively, promotes cyclin E-associated mammary tumorigenesis. The overall career development goal of the proposed work is to enable me to begin a productive, independent research career focused on understanding critical steps in the molecular pathogenesis of breast cancer. I recently started my laboratory at the Northwestern University Feinberg School of Medicine, where I joined as Assistant Professor in the Department of Medicine, Hematology/Oncology Division. At Northwestern, a comprehensive mentoring program is in place to support the goals of this proposed research as well as my career development.